Stem for electron discharge devices



March 1939. L. R. SHARDLOW 35L STEM FOR ELECTRON DISCHARGE DEVICES Filed Sept. 25, 1937 xxx 2 Y /4 H I4 /4 .5 D

I3 j E w I INVENTOR.

WEE/WE R. S/MRDLOW BY ATTORNEY.

* UNITED STATES PATENT OFFICE 2,151,809 STEM FOR ELECTRON DISCHARGE DEVICES Lawrence R. Shardlow, Arlington, N. 1., assignor, by mesne assignments, to Radio Corporation of America, New York, N.- Y., a corporation of Delaware Application September 25, 1937, Serial No. 165,677

' 4. Claims. (01. 250-275) My invention relates to electrical devices with sealed containers, such as electron discharge devices with metal envelopes, and particularly to stems for such devices.

For sealing the end of the conventional cupshaped metal envelope or shell, it has been proposed to use a stem comprising lead-in Wires sealed into a ceramic disc which is joined along its edge by glass to the rim of the metal shell,

as shown, for example, in the United States Patent 2,089,044 of Thomas, August 3, 1937. It has been found that differences of coefficient of expansion of the ceramic, glass, and metal shell may cause the metal to pull away from or shear the seal at the edge of the disc during sudden or wide temperature changes.

An object of my invention is an improved stem which is strong, easy to make, and which will remain vacuum tight during sudden and wide temperature changes.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, and one embodiment of the invention is described in the following specification and the accompanying drawing in which Figure 1 is a sectional perspective view of an electron discharge device having a stem made in accordance with my invention, Figure 2 is a detailed view of my improved stem, and Figure 3, shows a combined stem and base embodying the characteristic features of my invention;

The tube chosen for illustrating the characteristic features of my invention; and shown in Figure 1, comprises a metal envelope 1 with a flared rim or shoulder 2 and skirt portion 3 containing an electrode assembly 4 and is closed at its lower end with a header or stem made in accordance with my invention. The stem shown inthe drawing is illustrative of one embodiment of my invention and comprises a metal ring 5 U-shaped in cross section with two self-sustaining cylindrical portions or annuli 6 and i joined by a transverse section 8. The ring is conveniently stamped from sheet metal, such as chrome iron or nickel iron with a predetermined coefiicient of thermal expansion, and is provided with an outwardly extending flange 9.

The opening or hole of the inner cylinder 6 of the ring is closed by a fiat vitreous press or disc Ill of a ceramic material joined gas-tight to the inner cylinder by means of lead bore-silicate glass or glaze II, more fully described in co-pending application Serial No. 165,685, of John L. Gallup, filed September 25, 1937, and assigned to the same assignee as the present application. An exhaust tube i2, preferably of glass, may be sealed to the disc in registry with a central opening in the disc. Lead-in wires l3 extending through the disc are hermetically sealed in the disc at points on a circle, preferably concentric with the disc. The lead-in wires may be of the kind commonly used for radio tubes and lamps, such as dumet or an alloy which readily bonds with the sealing glass l4 fused between the walls of the opening and the sides of the lead-in wires. If desired, contact pins 15 may be embedded directly in the disc, secured by stapling as shown in Figure 3, and joined to the lead-in wires and sealed with a sealing flux or glass It in countersunk holes above the stapled pins.

To seal a ceramic disc by glass along its periphery to a surrounding metal ring and to prevent the metal ring from pulling away in a radial direction from the glass during changes in temperature, the metal should have a coefiicient of expansion that will shrink fit the metal ring on the disc and place a radial compressional strain on the glass. Preferably also, the coefficient of thermal expansion of the ceramic should be slightly higher than the coefiicient of expansion of the glass to eliminate tensional strains in the glass and prevent crazing or cracking of the glass.

Good results have been obtained by joining a chrome 104x 10- to 110 10- centimeters per centimeter per degree centigrade to a ceramic disc made according to my invention to have an expansion of about 100x 10' centimeters per centimeter per degree centigrade with an interposed sealing glass having a coefiicient of expansion of about 92 l0 A ceramimmade according to my invention, and having the desired coefficient of expansion, and the necessary strength and gas tightness, comprises powdered magnesium oxide, commercially known as magnesia, as the main constituent, combined with a small percentage of powdered clay and powdered acid magnesium metasilicate, commercially known as talc. I have found that feldspar may replace the tale. The coefficient of expansion of my ceramic has been found to be, within limits, proportional to the magnesia content of the ceramic. Ceramics made according to my invention with from 70% to 85% magnesia, and the balance consisting of about equal parts of clay and talc, have coefiicients of expansion of from about 8'7 10-' to about 124x 10 The magnesia, preferably commercial fused magnesia, is preferably ball-milled iron ring having an expansion' of until 100% of the material will pass a 250 mesh screen. The tale, commercially known as U. S. P. talc, and neutral to litmus, is powdered and screened through a 375 mesh sieve. The clay, preferably of the type commercially known as kaolin and containing less than 20% iron impurities is dried and ground to a particle size corresponding to about 325 or finer mesh screen. The powdered materials are mixed and a slip is conveniently prepared by adding 100 grams of the mixed powder to 150 cubic centimeters of carbon tetrachloride, and 8 grams of organic binder such as domestic parafiin in a 1 liter porcelain ball mill, containing 1000 grams of flint pebbles. To insure complete and uniform coating of the particles with a film of paraffin, the slip is ball-milled for two to three hours at a speed of to R. P. M., whereupon the slip may be poured from the ball mill and the carbon tetrachloride removed by slowly heating in air at about 110 C. for approximately twelve hours. The resulting aggregate may be crumbled while slightly warm and screened, preferably through a 40 mesh sieve. The resulting powder may now be pressed in steel molds into the desired size and shape, such as discs, by means of a plunger, preferably at a pressure of 5000 pounds per square inch. The paraffin binder may then be removed by firing the discs for a few minutes in air at a temperature of about 1100 C. The discs are then fired at high temperatures in a hydrogen atmosphere. While complete firing depends upon both temperature and time, good results have been obtained by firing a ceramic of 80% magnesia, 10% talc and 10% clay for about thirty minutes at 1350 C. followed by thirty minutes firing at about 1620 C. An automatically stoked hydrogen furnace sixty inches long has provided good results by moving trays containing the discs at a speed of one inch per minute through the first or front thirty inches of the furnace held at a temperature of 1350 C. and on through the rear thirty inches of the furnace maintained at 1620 C'.

By chemical analysis afinished ceramic made in accordance with my invention with 70% magnesia, 15% talc and 15% clay comprises 77% MgO, 6.1% alumina and 16.9% SlOz, and a ceramic made with magnesia, 7.5% talc, and 7.5% clay comprises 88.7% MgO, 3.0% alumina and 8.3% S102. It has been found that feldspar may replace the talc in my ceramic composition. By chemical analysis a ceramic made of 70% magnesia, 15% feldspar and 15% clay comprises 71.6% MgO, 8.8% alumina, 17.0% SiOz and 2.6% alkali oxide, and a ceramic of 85% magnesia, 7.5% feldspar and 7.5% clay comprises 85.9%! MgO, 4.4% alumina, 8.4% SiOz and 1.3% alkali oxide.

A finished ceramic disc made, for example, with 80% magnesia and the balance of equal parts of clay and talc consisting by chemical analysis of 84.8% MgO, 4.0% alumina and 11.2% SiO2 may then be sealed into a chrome-iron ring by coating the edge of the disc with a paste of water or alcohol and a ground low melting glass of the composition of 75% lead oxide PbO, 12% boron oxide B203, and 13% silica SiOz, made according to the Gallup application mentioned above, slipping the disc into the ring with the glass in contact with the inner cylinder of the ring, and heating to melt the glass, which thoroughly wets the ceramic and the metal and seals them together gas-tight. Alternatively, the ceramic disc may be joined with its lead wires and the metal ring by assembling the wires, disc, and ring, the disc preferably being held in place by its snug fit with the ring, depositing powdered glass around the lead wires and along the edge of the disc and heating preferably in a non-oxidizing atmosphere to melt and weld the glass to the parts. A film of glass at the sealing temperature to flow by capillary action into the annular space between the ring and the edge of the disc.

While the preferred embodiment of my invention has been described in detail, it is obvious to those skilled in the art that many modifications may be made without departing from the scope of my invention. My improved stem construction is strong, easy to make, and remains vacuum-tight throughout a long, useful life, the seal of the stem being unaffected by wide and sudden temperature changes.

I claim:

1. A stem for electron discharge devices comprising a metal ring with a thermal expansion between 104 10"" to 110x 10* centimeters per centimeter per degree centigrade, a ceramic disc with a thermal expansion of about 10- centimeters per centimeter per degree centigrade, and glass fiux with a thermal expansion of about 92 10- centimeters per centimeter per degree centigrade, the ceramic disc being sealed along its periphery to the metal ring by the glass.

2. In combination, a ceramic disc with a predetermined coeificient of thermal expansion, a selfsustaining metal ring with a coefficient of thermal expansion higher than the coeflicient of said ceramic disc, and glass with a coefficient of expansion lower than the coefficient of said disc adhesively joining the edge of the disc to the inner surface of the ring.

3. In combination, a ceramic disc with a predetermined co-eificient of thermal expansion, a thin glass coating with a co-efiicient of thermal expansion lower than the co-eificient of said disc adhesively sealed gas-tight to the edge of the disc,

and a strong metal ring with a co-eflicient of thermal expansion higher than the co-efilcient of said ceramic disc surrounding and compressing said disc and joined gas-tight to said glass coating.

4. A stem for an electron discharge device comprising a round ceramic body with a predetermined coeificient of thermal expansion, a metal ring with a coeflicient of thermal expansion higher than the coefficient of said ceramic body shrink fitted around said body, and a glass coating between said body and said ring adhesively joining the body and ring gas-tight.

LAWRENCE R. SHARDLOW. 

